Slope of geometry as `fac` input for cycles material
You can easily blend different shaders based on the angle of an object's face.

You can get the face's angle using the Normal output of the Geometry node. Separate out the axis you need (in my case the Z axis) using a Separate XYZ node. Then I use the ColorRamp node to fine tune the range I want the transition to happen in. Do note a RGB Curves node would work just as well, it is just up to which you prefer.


Here is what those nodes look like rendered.